The present invention regards the area of orthopedic surgery, and in particular a device for ensuring that prosthesis components are inserted correctly upon implantation of artificial hip joints, and to methods for doing the same.
An artificial hip joint has two main components; a prosthesis stem and a cup. One end of the prosthesis stem is provided either with a spherical ball head or a prosthesis neck on which can be placed a ball head, where the ball head is designed for a close, sliding fit in a spherical recess in the cup. Together, the prosthesis stem with the ball head and the cup will act as a ball joint to replace the natural ball joint.
The other end of the prosthesis stem comprises an elongated part designed to be attached to the hollow femoral canal in the patient's femur.
The cup is designed to be attached to the cavity on the patient's pelvis. The hemispherical shaped, recess in the cup is linked with an exterior surface designed to be attached to the pelvis, via a side face. The exterior surface may have various shapes, all according to the method of attachment to the pelvis and other choices made by the supplier. Several of the cups that are in use are shaped as an approximate hemisphere, where the outer hemispherical surface is designed to be cemented to the pelvis. The side face that connects the recess and the exterior surface may be flat or possibly inwardly sloping towards the recess, which is preferably approximately centered in the side face.
The prosthesis stem and the cup may be fixed to the femur and the pelvis respectively by using cement, or through a cement-free force fit. The invention may be used with both fixation techniques.
When replacing a worn out hip with a prosthesis, the head of the femur is replaced. This is done by cutting the neck of the femur and hollowing out the top of the femoral canal in order to make room for the elongated prosthesis stem that is either cemented into the hole or force fitted.
The cavity on the pelvis is milled out to receive the cup, which is then fixed either by means of cement or a force fit.
If the ball head is detachable, this is placed on the prosthesis stem before the ball head is placed in the cup, the joint is assembled by lifting the patient's leg up to a natural position and inserting the ball head in the recess in the cup, whereupon the incision is closed.
Such a prosthesis should give the patient a mobility that approximates that which is provided by the natural joint. However, as the joint capsule etc. is removed during the operation, it is possible for the patient to place the leg in a position outside its normal freedom of movement. This may cause the head of the prosthesis to jump out of the cup (luxation). Moreover, it is important that a “natural” movement of the joint does not cause the patient to get in a situation where the leg ends up in positions where the neck of the prosthesis rides on the edge of the cup. This happens through simple leverage. Luxation occurs in the case of between about 2 and 9% of all patients who have had a femoral prosthesis put in. If this happens, the patient must be anaesthetized before the joint is put back into place. Some patients must have a new operation. The risk of luxation is much greater in patients whose prosthesis components are assembled so as to have an incorrect mutual positioning, than in those where the mutual positioning of the components is correct.
The inventor has previously shown that an optimum mutual relationship between the prosthesis stem and the cup under experimental conditions (not published) results in a reduced risk of luxation because the patient can go through the everyday natural range of motion (ROM) without the parts of the prosthesis ending up in such mutual positioning so as to entail a risk of luxation.
The inventor has previously shown (not published) that the most adequate ROM is achieved by assembling both prosthesis components in a manner so as to give them a forward angle of about 15 degrees relative to the frontal plane of the body, while the cup forms an angle of 45 degrees with the horizontal plane. In medical terminology, forward angling is termed anteversion, whereas a backward angling is termed retroversion. An angle greater than 45 degrees relative to the horizontal plane when the patient is standing, is termed abduction, whereas an angle less than 45 degrees is called adduction.
The inventor has also previously shown (not published) that even though the optimum is to have each of the components angled forwards at 15 degrees, the result is nearly as good if the sum of the forward angling of the two components is 30 degrees. Thus a prosthesis joint where the cup is angled forwards at 10 degrees and the prosthesis stem is angled forwards at 25 degrees will result in a ROM for the patient that is nearly as adequate as if both components were angled forward at 15 degrees, the sum of the forward angling being 30 degrees for both cases.
During the fastening of the prosthesis stem accurate alignment of the prosthesis stem in the femur may be difficult in practice, especially if the stem is to be fastened cement free. Due to the shape of the internal channel in the femur, the prosthesis stem has a tendency to slide into the milled channel in the femur resisting to be forced into a specific angle.
Several solutions are known for insertion of the prosthesis stem in the cup and to ensure that the individual part is being fixed correctly.
A device for alignment and for holding the cup as it is cemented into the pelvis is known from U.S. Pat. No. 5,976,149. The temporary holding device for the cup is temporarily fixed to the pelvis during the cementation.
From GB 2,197,790 a device for assuring that the cup in an artificial hip joint is fixed with a predetermined anteversion and a predetermined angle to the horizontal plane, is known. The mutual angle between the parts in the prosthesis is not taken care of by using this device.
Instruments for insertion of the cup are described in EP 888,759 A1 and U.S. Pat. No. 5,540,697. These instruments are handles onto which the cup is fastened during the insertion but they do not have any means for assuring the correct position and direction of the cup. It is up to the individual surgeon and his experience to determine.
Several devices and means for assuring the alignment of the prosthesis stem during the insertion into the femur are known from EP 207 873, PCT/DE90/00715 and EP 865 776 A2. As mentioned this fixation is not critical. Additionally, these publications only describe devices and means for insertion of one of the prosthesis parts, i.e. the prosthesis stem, and do not describe any means to ensure an intended mutual angle between the cup and the prosthesis stem.
PCT/NO00/00299, having the same inventor as the present application, describes a tool to set the intended mutual angle between the prosthesis stem and the cup during the cementation of the cup in the pelvic cavity. The tool described may be locked relatively to the prosthesis stem and has one or more abutment surface(s) designed to rest against a surface of the cup so that the parts are locked relative to each other. Preferably the prosthesis is fixed to the channel I femur firstly, before the leg of the patient and the prosthesis stem is placed in a normalized position and is used to position the cup correctly. This device, however, may not be used by itself to assure the mutual positioning between the prosthesis parts when using cups to be mounted without the use of cement. Additionally it may only be used to assure that the parts of the prosthesis are positioned correctly relative to each other, but does not take into consideration the correct insertion relative to the patient.
Today there are no means available to ensure that the surgeon installs the prosthesis components with this correct mutual relationship. With today's methods therefore, this is done as judged by the eye. This judgement may be sufficient, especially for experienced surgeons who carry out a considerable number of this type of operation each year. It is estimated that surgeons who do less than 20 of these every year carry out 80% of all implantations of artificial hip joints. This number is not sufficient to get enough practice.
The bone coverage for the cup is often inferior when the cup is correctly mounted. The surgeon will often in cases like that choose to depart from the normally desired angle for the cup to get better bone coverage. In these cases it would be of great advantage if the surgeon could measure the actual angle and thus be able to choose the best compromise between angle and bone coverage.
It is therefore desirable to have a method and means that ensure a correct mutual positioning of the main parts of the prosthesis in order to reduce the possibility of errors, and thereby also reduce the risk of luxation with the resulting pain for the patient, and a possible second operation.
During insertion of the artificial hip joint it may also be desirable to adjust the length of the limb by inserting the prosthesis so that the effective length of the femur from the knee to the hip joint is lengthened or shortened. Additionally it may be desirable to adjust offset, i.e. the distance between the length axis of the femur and the sagittal plane of the body.
Today no good and reliable method or tool for reliable adjustment of the length of the limb or offset exists. An adjustment of the limb length may prevent consequential damage in the back, whereas an incorrect offset may reduce the vigor considerably as the attachment angle or the muscle leverage is incorrect. Thus, both errors may cause considerable pain and danger for consequential damage to the patient.